In recent times, a secondary battery refers to a device which converts external electric energy into the form of chemical energy and stores it therein, and then generates electricity as necessary. Such a secondary battery is also called the name of ‘rechargeable battery’ which means a battery capable of being charged many times. Typical secondary batteries include a lead storage battery, nickel cadmium (Ni—Cd) battery, nickel metal hydride (NiMH) battery, lithium ion (Li-ion) battery and a lithium ion (Li-ion) polymer battery. A secondary battery provides both an economical advantage and an environmental advantage as compared to a disposable primary battery.
Currently, a secondary battery is used for some applications requiring low electric power. For example, such applications include a device that helps starting of a car, portable system, instrument and a no-brake power supply system. Recently, development of wireless communication technology leads popularization of a portable system. In addition, there is a tendency to convert many conventional systems into wireless systems. Under these circumstances, there is an exploding demand for secondary batteries. Further, hybrid cars and electric cars have been commercialized with a view to preventing environmental pollution. Such next-generation vehicles adopt secondary batteries to reduce the cost and weight and to increase the service life.
In general, secondary batteries are generally provided as cylindrical, prismatic or pouch-type batteries. This is because secondary batteries are manufactured by installing an electrode assembly including an anode, cathode and a separator into a cylindrical or prismatic metal can or a pouch-type casing made of an aluminum laminate sheet, and then injecting an electrolyte to the electrode assembly. Therefore, a predetermined space for installing a secondary battery is required essentially. Thus, such cylindrical, prismatic or pouch-like shapes of secondary batteries undesirably function as limitations in developing various types of portable systems. As a result, there is a need for a novel type of secondary battery which allows easy deformation.
To meet such requirement, a linear battery having a significantly large ratio of length to sectional diameter has been suggested. Korean Laid-Open Patent No. 2005-99903 discloses a variable battery including an inner electrode, an outer electrode and an electrolyte layer interposed between the electrodes. However, such a battery has poor flexibility. In addition, since a cable-type secondary battery uses a polymer electrolyte to form an electrolyte layer, it is difficult to inject an electrolyte to the active material of an electrode. This results in the problems of an increase in resistance of a battery and degradation of capacity characteristics and cycle characteristics.
In addition, when forming a cable-type secondary battery, a non-uniform gap is generated between each electrode and the separator interposed between the inner electrode and the outer electrode. Due to such a gap, an electrolyte may not be introduced to the outer electrode active material layer smoothly, resulting in the problem of degradation of the quality of a battery.
Further, when a wire-type current collector is used for a cable-type secondary battery, resistance becomes high as compared to a sheet-type current collector. This may result in a problem of degradation of the quality of a battery.